Electronic equipment controlled by microcomputer requires a reserve power source to protect the microcomputer system from memory loss resulting from a momentary main power shut-down. A reserve power source for microcomputers must be small in size yet have sufficient capacitance to store enough electric charge to protect the computer memory from erasure during momentary power failure.
New capacitors have been developed in an attempt to provide a reserve power supply having a small size and increased capacitance, including an activated carbon electrode and an ionically conductive sulfuric acid electrolyte (see Sanada and Hosokawa, Electric Double Layer Capacitor "Super Capacitor", NEC Research and Development, No. 55 Oct. 1979, pages 21-27). The unit cell voltage for the "Super Capacitor" is 1.2 volts. Accordingly, to achieve the required voltage for microcomputer memory protection of, say 5 volts D.C., it is necessary to stack several unit cells.
This assignee, similarly, has developed a new capacitor having a high surface area carbon-electrolyte electrode, a lead metal-electrolyte electrode, and an ionically conductive electrolyte to achieve a capacitor having a high capacitance and a small volume, Phillips et al. U.S. Pat. No. 4,438,481. Again, however, the maximum voltage each cell can support is only about 1.25 V D.C. making it necessary to stack a plurality of cells in series to achieve the required operating voltage of, for example, 3 to 5 V D.C.
It is well known in the art of capacitor manufacturing that the necessity of stacking capacitors is time consuming, may require selection and testing of appropriate unit cells to be stacked together and is otherwise economically inefficient. However, in spite of the obvious drawbacks and long felt need associated with stacking capacitors to achieve the needed operating voltage in a capacitor having a small volume, others have been unable to provide a single unit cell having the required small volume capable of microcompuer memory protection. One area of pursuit has been to provide various capacitor electrode materials having a very large surface area/volume ratio to achieve a high capacitance at a small volume, such as the activated carbon disclosed in the Phillips U.S. Pat. No. 4,438,481. While the volumetric efficiency is substantially improved in the capacitor described in the Phillips et al. '481 patent, each cell has a maximum applied voltage of only about 1.25 V so that four cells must be stacked in series to provide a typical voltage rating of 5 V D.C. as a back-up power source for a typical semiconductor memory device.
Other anode materials, such as the oxide coated valve metals having relatively thick oxide coatings are known to support higher voltages, but the capacitance of such materials is limited because of the smaller surface area and the thickness of the oxide coating. Aluminum and tantalum electrolytic capacitors are typical examples of the use of higher voltage supporting, less capacitance, capacitor electrodes, having relatively thick oxide coatings.
Any reduction in the thickness of the anode oxide coating on these electrodes, while increasing the capacitance of the device, also decreases the maximum voltage that the capacitor can support so that more than one of the capacitors must be stacked in series to achieve necessary voltage rating, at a small volume.